Lithium Ion Battery Charging

Basics

These
remarks apply equally to lithium ion and lithium polymer batteries. The
chemistry is basically the same for the two types of batteries, so charging
methods for lithium polymer batteries can be used for lithium-ion
batteries.Charging lithium iron phosphate 3.2 volt cells is identical, but
the constant voltage phase is limited to 3.65 volts.

The lithium ion
battery is easy to charge. Charging safely is a more difficult. The basic
algorithm is to charge at constant current (0.2 C to 0.7 C depending on
manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the
voltage at 4.2 volts until the charge current has dropped to 10% of the initial
charge rate. The termination condition is the drop in charge current to 10%.
The top charging voltage and the termination current varies slightly with the
manufacturer.

However, a charge timer should be included for
safety.

The charge cannot be terminated on a voltage. The capacity
reached at 4.2 Volts per cell is only 40 to 70% of full capacity unless charged
very slowly. For this reason you need to continue to charge until the current
drops, and to terminate on the low current.

It is important to note that
trickle charging is not acceptable for lithium batteries. The Li-ion chemistry
cannot accept an overcharge without causing damage to the cell, possibly
plating out lithium metal and becoming hazardous.

Float charging,
however, is a useful option. The safety issue with keeping the battery on
constant charge is that if the charger should somehow go haywire and apply a
higher voltage there could be problems. And, so the logic goes, the shorter the
charger is turned on the less likely the charge will go haywire while connected
to the battery. However, there is another safety method, the battery protection
board, which should be included either on the battery or in other circuitry
between the battery and the charger. The BPB (also known as PCB for "protection
circuit board") or other battery management circuit will stop the charge if the
voltage gets too high.

Charging Lithium ion batteries at slow
rates

When the charge rate during the constant current phase
is low, the charger process will spend less time during the constant voltage
tail. If you charge below about 0.18 C, the cell is virtually full when the 4.2
volts is reached. This can be used as an alternative charge algorithm. Just
charge below 0.18C constant current and terminate the charge when the voltage
reaches 4.2 volts per cell.

Safety

Every lithium
ion battery pack should have a method of keeping the cell balanced and
preventing them from being over-discharged. This is usually done with a safety
board which monitors the charge and discharge of the pack, and prevents
dangerous things from happening. The specifications of these safety boards are
dictated by the cell manufacture, and may include the following:

Reverse polarity protection

Charge temperature--must not be charged when temperature is
lower than 0° C or above 45° C.

Charge current must not be too high, typically below 0.7
C.

Discharge current protection to prevent damage due to short
circuits.

Charge voltage--a permanent fuse opens if too much voltage is
applied to the battery terminals